In optical communication systems a single optical fiber may be used to carry multiple optical signals. Each of the optical signals may be modulated on a separate associated wavelength referred to as a channel. The optical signals may be multiplexed to form a wavelength division multiplexed (WDM) signal that is transmitted on the optical fiber. WDM systems may have a high traffic capacity. For example, a WDM system may carry 100 channels or more at 10 to 100 gigabits per second (hereinafter Gb/s) or higher.
WDM optical communication systems may be provided in a trunk and branch configuration including a trunk path extending between trunk terminals and branch paths coupled to the trunk path by associated branching units and extending between the trunk path and associated branch terminals. The system may be configured to provide independence between the branch terminals while maximizing transmission capacity on the trunk and branch paths.
Some trunk and branch optical communication systems do not require maximum capacity on the trunk and branch paths. For example, some optical communication systems may include a large number, e.g. sixty-four or more, of branch paths connecting two shore landing stations to associated oil platforms and other offshore resources (referred to herein as nodes) and may thus place a high value on the number of supported connections.
The reliability of connectivity from each node to the shore may be critical in such applications and may be achieved by sending a wavelength from each node in an “east” direction toward one shore station, while the same data is replicated on a wavelength going “west” to the other shore station. The terms “east” and “west” are used herein to describe opposite directions of transmission in an optical cable and not necessarily to describe compass direction.
One challenge in these systems may be the overall loss on the trunk path resulting from a large number of nodes and associated branching units. Each branching unit (BU) coupled to the trunk path for supporting a branch path and an associated node contributes loss, e.g. 3 dB or higher, to the overall loss on the trunk path. The loss may require compensation by addition of optical amplifiers to the trunk path. Each optical amplifier may compensate for the loss associated with only a few of the nodes coupled to the trunk path.